Method of manufacturing silicon nanowire array

ABSTRACT

Provided is a method for manufacturing a silicon nanowire array comprising the steps of: positioning plastic particles separated apart from one another in a uniform random pattern on a silicon substrate; forming a catalyst layer between the plastic particles; removing the plastic particles; vertically etching portions of the silicon substrate that contact the catalyst layer; and removing the catalyst layer. The present invention provides a simple and cost-effective process, enables mass-production through large surface area processing, enables the manufacture of nanowire even at a site having limited resources, and enables the structures of nanowire to be individually controlled.

TECHNICAL FIELD

The present invention relates to a method for manufacturing a siliconnanowire array.

BACKGROUND ART

A nanowire is one of various semiconductor nanostructures and refers toa wire structure that has nanoscale dimensions. Generally, a nanowireincludes a wire having a diameter of less than 10 nm to several hundrednanometers.

Methods for manufacturing such nanowires are divided into three majortypes.

First, there has been proposed a method in which a photoresist ispatterned into nanoscale dimensions using an e-beam lithographyapparatus, followed by etching silicon into nanoscale dimensions usingthe patterned photoresist as a mask, thereby manufacturingtwo-dimensional silicon nanowires.

However, such a conventional method for manufacturing silicon nanowiresis unsuitable for mass production due to high fabrication costs.

Second, there has been proposed a vapor-liquid-solid (VLS) method whichis a self-assembly method wherein a metal catalyst having nanoscaledimensions is formed, followed by introduction of a reaction gas (SiH₄)while maintaining a high temperature of about 950° C., thereby growingtwo-dimensional silicon nanowires.

However, this method has difficulty in controlling a structure ofnanowires and cannot control a direction in which silicon nanowires aregrown.

Third, there has been proposed etching using a solution process. Etchingusing a solution process can provide reduction in time and cost, ascompared with the self-assembly method. In the method for manufacturingsilicon nanowires using a solution process, a hexagonal lattice patternusing nanostructures has been mainly used to precisely control geometricparameters (diameter, height, density, and the like) of siliconnanowires. However, it is difficult for this method to achieveindividual control of parameters of the nanowires and to fabricatelarge-area nanowires.

DISCLOSURE Technical Problem

It is an aspect of the present invention to provide a method formanufacturing a nanowire array, which is capable of independentlycontrolling geometric parameters (diameter, length, density, location,and the like) of nanowires and is cost-effective and allows massproduction.

Technical Solution

In accordance with one aspect of the present invention, there isprovided a method for manufacturing a silicon nanowire array,comprising: placing plastic particles separated from each other in auniform random pattern on a silicon substrate; forming a catalyst layerbetween the plastic particles; removing the plastic particles;vertically etching a portion of the silicon substrate contacting thecatalyst layer; and removing the catalyst layer.

Advantageous Effects

According to the present invention, it is possible to provide a methodfor manufacturing nanowires, which provides a simple process, iscost-effective, allows mass production by large-area processing, andallows fabrication of nanowires even in a resource-constrainedenvironment. In addition, the present invention allows individualcontrol of the structure of the nanowires and is thus expected to beapplied to various fields using the nanowires, such as a solar energyindustry including electronic devices and solar cells, biosensors, andthe like.

DESCRIPTION OF DRAWINGS

FIG. 1 is a flow diagram showing a method for manufacturing a nanowirearray according to one embodiment of the present invention.

FIG. 2 is a view illustrating a mechanism of chemical etching with ametal catalyst according to one embodiment of the present invention.

FIG. 3 shows images showing diameters of silicon nanowires dependingupon the size of polystyrene beads according to one embodiment of thepresent invention.

FIG. 4 shows images showing heights of silicon nanowires depending uponetching time according to one embodiment of the present invention.

FIG. 5 shows images showing density of a silicon nanowire array at eachdensity of polystyrene beads according to one embodiment of the presentinvention.

FIG. 6 shows TEM and EDX analysis images of silicon nanowiresmanufactured according to one embodiment of the present invention.

FIG. 7 is a view showing a patterning process of a vertical siliconnanowire array (vSiNWA) according to one embodiment of the presentinvention.

FIG. 8 is an image showing a core-shell structure of FeO_(x) formedusing silicon nanowires as a template.

FIG. 9 is a graph showing distributions of plastic particles havingdifferent sizes depending upon the number of stacked polymer layers.

BEST MODE

As used herein, the term “nanoscale” or “nano” may refer to a size of 1nm to less than 1000 nm, without being limited thereto.

Hereinafter, embodiments of the present invention will be described withreference to the accompanying drawings such that those skilled in theart to which the present invention pertains can easily realize thepresent invention.

Recently, nanoscale materials have emerged as a very important field ofstudy due to their new physicochemical properties such as uniqueelectrical, optical, and mechanical characteristics. In addition,studies on nanostructures reported so far show that nanomaterials havethe potential to be new materials for optical devices in the future.Particularly, a nanoscale device has increased surface/volume ratio dueto small size thereof and thus allows predominant electrochemicalreactions on a surface thereof to be applicable to various sensors. Inparticular, a vertical silicon nanowire array (vSiNWA) is in thespotlight because of useful electrical properties, large surface area,quantum confinement effect, biocompatibility, and the like.

However, since most nanoscale devices are difficult to apply topractical use due to difficulty in artificial manipulation, as analternative, easier to manipulate materials for nanoscale devices, suchas nanowires, are being studied. Nanowires can be widely applied tovarious fields such as biosensors, optical devices including lasers,transistors, memory devices, and the like.

Nanowires are currently manufactured by a growth method using catalysts.

In such a nanowire fabrication method, nanowires are formed to apredetermined length, followed by removal of the catalysts. However, inthis case, it is difficult to freely control diameter, length, density,location, and the like of the nanowires.

The present invention has been conceived to solve such a problem in theart and is aimed at providing a method for manufacturing a verticalsilicon nanowire array through solution-based chemical etching. To thisend, in accordance with one aspect of the invention, there is provided amethod for manufacturing a silicon nanowire array which includes:placing plastic particles separated from each other in a uniform randompattern on a silicon substrate; forming a catalyst layer between theplastic particles; removing the plastic particles; vertically etching aportion of the silicon substrate contacting the catalyst layer; andremoving the catalyst layer, as shown in FIG. 1.

Although crystalline silicon (Si) is selected as a material fornanowires in this embodiment, it should be understood that the presentinvention is not limited thereto, and amorphous silicon (a-Si) andpolycrystalline-Si may also be formed into large area nanowires and anano-pattern through a similar solution process.

In the present invention, a process of manufacturing a nanowire arrayusing silicon will be mainly described.

First, plastic particles are arranged to be separated from each other ona silicon substrate. A polymer layer may be formed on the siliconsubstrate before arranging the plastic particles on the siliconsubstrate. To this end, a solution containing a cationic polymerelectrolyte to the silicon substrate and a solution containing ananionic polymer electrolyte to the silicon substrate are alternatelyapplied, thereby forming a lamellar self-assembled polymer layer in theform of layer-by-layer films. This process may be repeated severaltimes. The cationic polymer electrolyte may be selected from the groupconsisting of polyarylamine chloride, polyethyleneimine,polydimethyldiallyl amide, polylysine, and combinations thereof, withoutbeing limited thereto, and the anionic polymer electrolyte may beselected from the group consisting of polystyrene sulfonate, polyacrylicacid, polyvinyl sulfate, heparin, and combinations thereof, withoutbeing limited thereto.

When a negatively charged substrate such as a silicon substrate is used,a solution containing a cationic polymer electrolyte charged oppositethe charge of the substrate may be applied to the substrate first andthen a solution containing an anionic polymer electrolyte may beapplied. Here, preferably, a polymer electrolyte-containing solutionapplied last is charged opposite the charge of the plastic particles. Inthis way, a single or plurality of polymer layers may be formed on thesubstrate.

Such polymer electrolyte-containing solutions may be applied to thesubstrate by any suitable method known in the art, for example, bydipping the substrate in the solutions, without being limited thereto.

The polymer layer formed on the substrate serves as an adhesive throughwhich plastic particles are attached to the substrate. In addition, thepolymer layer helps the plastic particles to be uniformly spaced fromeach other by providing electrostatic force between the plasticparticles. Referring to FIG. 9, there are shown distributions of plasticparticles having a size of 100 nm, 150 nm, 200 nm or 350 nm, wherein theplastic particles are placed on the substrate to be separated from eachother. When the polymer layer was formed in a total of 1, 3, 5, and 7layers, distribution of the plastic particles became more uniform withincreasing number of stacked polymer layers. Therefore, it can be seenthat the polymer layer assists in uniform distribution of the plasticparticles.

The plastic particles preferably have a spherical shape and may beformed in various shapes, as needed. The plastic particles may bescattered in a granular form on the silicon substrate with the polymerlayer formed thereon, or may be applied to the substrate by spin coatinga mixture of the plastic particles with a certain solution. The plasticparticles may be arranged in a periodic or aperiodic manner As a result,the plastic particles may be placed apart from one another in a uniformrandom pattern.

The plastic particles may be selected from the group consisting ofpolyethylene, polypropylene, polystyrene, polyethylene terephthalate,polybutylene terephthalate, polycarbonate, polymethyl methacrylate,polyphenylene oxide, and polyacetal, without being limited thereto, andsuch plastic particles preferably have a nano sc ale diameter.

When the plastic particles are dispersed in position, the polymer layeror solution component is removed by oxygen plasma treatment. Then, acatalyst layer is formed between the plastic particles by depositing thecatalyst layer on the substrate with the plastic particles attachedthereto. Specifically, the catalyst layer may be formed by sputtering,electron-beam evaporation, vacuum deposition, chemical vapor deposition,physical vapor deposition, atomic layer deposition (ALD), and the like.

Through the deposition process, the catalyst layer is formed in a spacebetween the plastic particles as well as on exposed surfaces of theplastic particles. When the plastic particles are then removed, thecatalyst layer remains absent at positions at which the plasticparticles have been present.

The catalyst layer may include silver (Ag), gold (Au), platinum (Pt),copper (Cu), or a combination thereof.

In the present invention, a nanowire array is formed by chemical wetetching using a metal catalyst. As used herein, wet etching refers to amethod in which a material to be etched is brought into contact with anetching solution, thereby etching the material through chemicalreaction.

The etching solution may include an acid and a peroxide. A typicalexample of the acid may include hydrofluoric acid (HF) and a typicalexample of the peroxide may include hydrogen peroxide (H₂O₂). Timerequired for etching may also be adjusted by appropriately adjustingconcentrations of the acid and the peroxide in the etching solution.

In a portion of the silicon substrate contacting the catalyst layercontaining the metal catalyst, holes are formed in silicon as hydrogenperoxide is reduced due to the metal catalyst. A region where such holesare abundant is exposed to the acid to be dissolved.

Now, a reaction mechanism of the above process will be described indetail with reference to FIG. 2.

(Cathode Reaction)

H₂O₂+2H⁺+2H₂O

2H⁺+2e^(−→H) ₂

(Anode Reaction)

Si+2H₂O→>SiO₂+4H⁺+4e⁻

SiO₂+6HF→H₂SiF₆+2H₂O

Si+4HF→SiF₄+H⁺4e⁻

When the reactions as above are repeated, silicon under the catalystlayer is etched by dissolution and a metal in the catalyst layer fallsonto underlying silicon.

After etching is completed by repetition of this process, the metal isfinally removed, thereby obtaining a desired nanowire array.

Here, the metal in the catalyst layer may be removed using aqua regia,which is a mixture of nitric acid and hydrochloric acid.

In the present invention, structural parameters of nanowires may becontrolled by appropriately adjusting process conditions. Nanowires withindividually controlled structural parameters, i.e. diameter, height,density, location, and the like, may be utilized in observing neuronalinterfaces.

By way of an example of methods for controlling structural parameters,the plastic particles may be controlled in size so as to adjust diameterof nanowires. As shown in FIG. 3, it can be seen that nanowires having adiameter of 100 nm ((a) in FIG. 3), 150 nm ((b) in FIGS. 3), and 240 nm((c) in FIG. 3) were obtained using plastic particles having a size of100 nm, 150 nm, and 240 nm, respectively.

In addition, height of nanowires, i.e. length of the nanowires may beadjusted by controlling the period of time for which etching isperformed using an etching solution. Here, the height or length ofnanowires increases with increasing etching time. As shown in FIG. 4,the nanowires had an average height of less than 0.9 μm, less than 1.4μm, and less than 1.8 μm for an etching time of 1 minute ((a) in FIG.4), 2 minutes ((b) in FIGS. 4), and 3 minutes ((c) in FIG. 4),respectively.

Further, density of a nanowire array may be adjusted by controlling thedistance between the plastic particles. In terms of fabrication process,density of the plastic particles in a solution containing the plasticparticles is controlled by adjusting the dilution ratio of the solutionwith deionized water, followed by application of the solution, wherebythe distance between the applied plastic particles can be controlled.

As shown in FIG. 5, it can be seen that, by appropriately adjusting thedilution ratio of the solution containing the plastic particles withdeionized water, it is possible to fabricate nanowire array substrates,the density of which is controlled in density such that the number ofnanowires per sectional area of 100 μm² becomes 350 or less ((a) in FIG.5), 700 or less ((b) in FIGS. 5), and 1400 or less ((c) in FIG. 5),respectively.

The nanowire array according to the present invention may have aperiodic structure formed in a predetermined pattern. Such a structuremay be realized by combination of metal assisted chemical etching (MACE)and photolithography.

Now, a process of etching in a pattern will be described in detail withreference to FIG. 7.

A photoresist is applied to a silicon substrate by spin coating,followed by photolithography using a desired pattern mask. A polymerlayer is formed on the patterned silicon substrate by layer-by-layerself-assembly, and the plastic particles having a desired size areplaced thereon. Then, a metal catalyst layer is deposited thereon by themethod as described above, followed by chemical reaction in an etchingsolution, thereby forming a vertical nanowire array structure only at adesired pattern portion.

In addition, the nanowire array according to the present invention mayhave an aperiodic nanostructure by randomly dispersing and coating theplastic particles. In other words, nanospheres may be uniformlydispersed and coated in a random manner rather than in a hexagonallattice pattern by combination of layer-by-layer self-assembly andnanosphere lithography to individually control geometric parameters ofnanowires, thereby manufacturing a nanowire array having an aperiodicarrangement.

Results of characterization as shown in FIG. 6 were obtained bytransmission electron microscopy (TEM) and energy-dispersive X-rayanalysis (EDX) of manufactured silicon nanowires.

By TEM, crystallinity and surface structure of the manufactured siliconnanowires can be confirmed. It can be seen that surfaces of thenanowires have an amorphous shape and have many fine pores, which arecharacteristics in fabrication of nanowires by etching. The nanowireshave a crystalline shape inside thereof and exhibit [100] crystallinityinherent to a silicon wafer. In addition, since it is confirmed that themanufactured silicon nanowires contain silicon and oxygen atoms bycomponent analysis through EDX, it can be inferred that the siliconnanowires have a silicon oxide layer on surfaces thereof.

Further, at least one material layer may be coated onto outer surfacesof silicon nanowires, which are used as a template, thereby obtaining acore-shell nanowire structure capable of providing predeterminedoptical, electrical, magnetic, mechanical and chemical functions,wherein the material layer is formed of, for example, a metal such asSi, Ge, Cu, Ni, Cr, Fe, Ag, Ti, Co, Zn, Mg, Pt, Pd, Os, Au, Pb, Ir, Mo,V, and Al, an alloy thereof, a metal oxide such as SnO₂, Cr₂O₃, Fe₂O₃,Fe₃O₄, FeO, NiO, AgO, TiO₂, Co₂O₃, Co₃O₄, CoO, ZnO, PtO, PdO, VO₂, MoO₂and PbO, a polymer such as polyimide, or a combination thereof having amultilayer structure such as Ti/TiO₂. Such a material may be coated ontothe silicon nanowires by any typical thin-film deposition method knownin the art, such as chemical vapor deposition, atomic layer deposition,and sputtering.

In some embodiments, after coating of the silicon nanowires with atleast one material layer, the silicon nanowires may be removed, therebyobtaining a nanotube array structure in which the material layer is leftalone. As such, the silicon nanowires used as a template may be presentinside the nanotube array structure or may be removed by dry etchingwith plasma or wet etching with HF depending upon device fabricationmethod.

A method for forming a core-shell nanowire structure is as follows. Inthe method, a process of substituting a surface of a silicon nanowiretemplate formed by the method described above is needed. A FeO,,solution is prepared by a sol-gel process, followed by placing asurface-treated silicon nanowire template into the solution. Then, thesilicon nanowire template is treated using oxidizing water, therebymanufacturing a core-shell structure of FeO_(x) taking the form of thesilicon nanowires, as shown in FIG. 8.

What is claimed is:
 1. A method for manufacturing a silicon nanowirearray, comprising: placing plastic particles separated apart from oneanother in a uniform random pattern on a silicon substrate with alamellar self-assembled polymer layer formed thereon; forming a catalystlayer between the plastic particles; removing the plastic particles;vertically etching portions of the silicon substrate contacting thecatalyst layer; and removing the catalyst layer.
 2. The method accordingto claim 1, wherein the lamellar self-assembled layer is formed byalternately applying a solution containing a cationic polymerelectrolyte and a solution containing an anionic polymer electrolyte tothe silicon substrate.
 3. The method according to claim 2, wherein thecationic polymer electrolyte is selected from the group consisting ofpolyarylamine chloride, polyethyleneimine, polydimethyldiallyl amide,polylysine, and combinations thereof.
 4. The method according to claim2, wherein the anionic polymer electrolyte is selected from the groupconsisting of polystyrene sulfonate, polyacrylic acid, polyvinylsulfate, heparin, and combinations thereof.
 5. The method according toclaim 1, wherein the plastic is selected from the group consisting ofpolyethylene, polypropylene, polystyrene, polyethylene terephthalate,polybutylene terephthalate, polycarbonate, polymethyl methacrylate,polyphenylene oxide, and polyacetal.
 6. The method according to claim 1,wherein the catalyst layer comprises silver (Ag), gold (Au), platinum(Pt), copper (Cu), or a combination thereof.
 7. The method according toclaim 1, wherein forming the catalyst layer is performed by deposition.8. The method according to claim 1, wherein etching is performed by wetetching.
 9. The method according to claim 8, wherein the wet etching isperformed using a solution containing an acid and a peroxide.
 10. Themethod according to claim 1, wherein a diameter of silicon nanowires isadjusted by controlling the size of the plastic particles.
 11. Themethod according to claim 1, wherein a density of the silicon nanowirearray is adjusted by controlling a distance between the plasticparticles.
 12. The method according to claim 1, wherein a height ofsilicon nanowires is adjusted by controlling a period of time for whichetching is performed.